What are the two major contributors to sprawl?

Some possible answers are: unrestricted growth, unlimited use of cars and building or extensive road networks. Unrestricted growth in a city means that there is often insufficient housing near the city centre, and people move outwards to find affordable housing. The use of cars along with extensive road networks has also provided people the ability to commute longer distances to work, resulting in more outlying residential areas. I would add lack of housing planning as another factor leading to housing prices near the city centre being unaffordable for most middle-class families.